Immunologic Hyporesponsiveness to Serogroup C but Not Serogroup A following Repeated Meningococcal A/C Polysaccharide Vaccination in Saudi Arabia

In Saudi Arabia, vaccination with the meningococcal A/C polysaccharide (MACP) vaccine is advised every 3 years. A clinical outcome study was performed to test the effect of repeat vaccination with the MACP vaccine on the immune responses among Saudi nationals who live in the Makkah and Jeddah areas. Subjects (n = 230) aged 10 to 29 years were selected: 113 subjects with two or more prior vaccinations with the MACP vaccine, 79 subjects with one prior vaccination with the MACP vaccine, and 38 subjects naïve to vaccination with the MACP vaccine. All subjects received the MACP vaccine in 2002, and serum bactericidal antibody (SBA) titers were measured before and 1 month after vaccination with the MACP vaccine. For serogroup C, geometric mean SBA titers 1 month following vaccination with the MACP vaccine were 708.6 (95% confidence interval [CI], 217.5 to 2,308.9) for those naïve to prior vaccination with the MACP vaccine, and they were (P <0.0001) than 25.0 (95% CI, 12.4 to 50.2) for those who had received one prior vaccination with the MACP vaccine and 32. 4 (95% CI, 18.7 to 56.4) for those who had received two or more doses of the MACP vaccine. For serogroup A, the geometric mean SBA titer 1 month after receipt of the MACP vaccine was 1,649.3 (95% CI, 835.2 to 3,256.9) for those naïve to prior vaccination, and the titers were lower (P = 0.67) than 2,185.7 (95% CI, 1,489.4 to 3,207.7) for those who had received one prior dose of the MACP vaccine and significantly lower (P = 0.042) than 3,540.8 (95% CI, 2,705.2 to 4,634.5) for those who had received two or more doses of the MACP vaccine. For serogroup C, the proportions of nonresponders (SBA titers,